This is a pretty neat trick. You trim back the plastic sleeve on the negative end of the battery on one side - not all the way around. You pry up the end of the battery slightly, and put one end of the bent staple (or paper clip, or wire, or whatever) in the gap, under the paper insulator. Then you use a piece of char cloth or high quality tinder to press the free end of the staple against the negative contact of the battery. If your tinder is good, it will produce enough heat to get a coal. Here is a link to the video: (I did not make this video, I found it)
http://www.wonderhowto.com/how-to-su...staple-222300/
If you are in the woods you may likely have a camera or other device that requires batteries. I imagine that this could be done with AAA, C, or D batteries as well. If you do not have a staple, you are like to be able to come up with a usuable piece "wire-like" metal. The only drawback I can see, and it is a big one, is the necessity of having char cloth or extremely good tinder on hand.